Service Revenue (Summary of Changes in Deferred Costs to Obtain and Fulfill Contracts) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Nov. 30, 2019	Nov. 30, 2018	Nov. 30, 2019	Nov. 30, 2018
Capitalized Contract Cost [Line Items]
Amortization			$ 92.4	$ 89.5	[1]
Costs To Obtain A Contract [Member]
Capitalized Contract Cost [Line Items]
Beginning balance	$ 460.0	$ 448.5	464.3	455.0
Capitalization of costs	37.4	37.9	73.3	70.3
Amortization	(40.5)	(39.1)	(80.7)	(78.0)
Ending balance	456.9	447.3	456.9	447.3
Costs To Fulfill A Contract [Member]
Capitalized Contract Cost [Line Items]
Beginning balance	66.3	65.7	66.1	65.4
Capitalization of costs	6.0	5.9	12.0	11.9
Amortization	(5.9)	(5.8)	(11.7)	(11.5)
Ending balance	$ 66.4	$ 65.8	$ 66.4	$ 65.8

[1] Amounts have been adjusted to reflect adoption of Accounting Standards Update (“ASU”) No. 2016-18, “Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the FASB Emerging Issues Task Force).”